Does your undergraduate school have a Premedical Committee? If yes, you'll get a committee letter from them.
1. Many medical schools may be okay with a science LOR written by an engineering professor BUT some schools are much more strict and inflexible, and may require only "BCPM" science LORs.
2. As a medical student, you're going to be studying, applying, integrating, and presumably memorizing different types of science information in your pre-clinical years (and not studying engineering or statistics). So, medical schools want to make sure you can successfully manage the academic rigors associated with your medical school studies. As a general rule, that's BCPM material. Enough said.
3. To be on the safe side, you can email the medical schools to which you plan to apply and specifically ask them if they will accept a "science LOR" from this particular STEM professor. Yes or no? Double-check ahead of time.
4. In so doing, you will have a "paper trail" that you can keep in your email file - just in case anyone objects to that science LOR, on some inflexible LOR classification. This level of inflexibility can occur with some schools - so do whatever you need to do ahead of time to prevent that type of pesky science LOR problem from affecting your medical school application.
TL/DR: suggest you double-check with each medical school ahead of time to make sure they will accept a "science LOR" from your STEM professor (engineering/statistics) - just to be on the safe side. Double-check ahead of time.